FO, as an alternative membrane process in wastewater treatment, has attracted increasing interest in recent years. FO is the process in which water molecules pass through a semipermeable membrane, which separates two solutions, as shown in Figure 1. This transport and movement of molecules takes place due to the osmotic pressure difference (Δπ) which is the driving force in this phenomenon, as opposed to pressure-driven membrane processes. Thus, water is permeated passing through the membrane from the lowest solution concentration, FS, to the highest solute concentration solution, DS, while other solutes molecules are rejected [19,43]. FO has been investigated in various applications, such as seawater desalination [43], power generation [44], food processing [45], and wastewater treatment [46,47].

The beginning of the interest in FO dates back to the 18th century [48,49], while interest in this field has increased due to the commercialization of membranes designed for this process [2]. Figure 2 shows the rising interest in membranes of FO in the last 20 years by analyzing the number of publications on the topic.

Forward osmosis membranes are of interest if they have elevated water permeability while keeping salt retention high. In addition, they must present low concentration polarization, which is a phenomenon that, in the forward osmosis process, causes the osmotic pressure to decrease, leading to a reduction in the flow of water through the membrane. Furthermore, good chemical and mechanical stability to withstand working conditions is required [50].

FO membrane modules can be classified into plate and frame, spiral wound, tubular, hollow fiber, and flat sheet, according to the various geometric structures. The most used FO membrane modules are flat sheets and especially hollow fibers because these configurations require little space and are capable of separating large volumes, which are advantageous factors when compared with other membrane module configurations [51].

The most common FO commercial membranes, with respect to the material used, are cellulose acetate/triacetate (CA/CTA)-based membrane and thin-film composite (TFC) membranes of polyamide, polysulfone, or polyester layers [52,53,54,55,56,57]. A recent study proposed a classification of the emerging FO membranes into four categories according to their fabrication methods: cellulose acetate (CA), thin-film composites (TFCs), polybenzimidazole (PBI), and aquaporin (AQP), with TFCs the most competitive according to their properties [58].

The first commercialized FO membranes, i.e., CA/CTA membranes, have advantages such as good mechanical resistance, low tendency to fouling, good permeate fluxes, and high resistance to chlorine [59]. However, the operation pH range (3–8) is somewhat limited. To improve the characteristics of CA/CTA membranes, TFC membranes with a pH range of 2–11 and with higher permeate fluxes have been produced [2,51].

In addition to commercial membranes, numerous recent studies tried to modify the structure of the support layer using different methods or additives such as silica, graphene, zeolite, and TiO₂ to improve the properties of commercial membranes [51,60,61,62,63].

Various industrial companies offer FO membranes and commercial FO systems. Initially, the pioneering company for the supply of FO membranes was Hydration Technology Innovations (HTI) founded in 1986 in Albany (NY, USA). Later, another company called Oasys Water Inc. began to commercialize FO modules in the year 2010 in Cambridge (MA, USA). Another firm that manufactures FO membranes is FTS H2O™, also working in Albany (USA), specializing in CTA membranes in flat sheets. Next, the company Aquaporin Inside™ introduced FO membranes with aquaporin proteins that are highly selective, facilitating the transport of water molecules. These thin-film composite membranes are available in both flat sheet and hollow fiber configurations. In addition, Aquaporin A/S, a developer of these biomimetic membranes based in Lyngby (Denmark), recently signed a development agreement with another leading tubular membrane manufacturing company called Berghof Membrane Technology based in Leeuwarden (the Netherlands) to launch new membranes. Other companies have manufactured or have collaborated in the manufacture of FO modules such as Toray, Toyobo, Koch membrane systems, and Porifera, as well as some intermediary companies for marketing this type of module such as Sterlitech [64]. It should be noted that the supply of this type of FO membranes has facilitated studies and research related to FO that otherwise would have been much less developed today.

The operating conditions significantly affect the performance of FO. Therefore, their optimization is necessary to make the FO process more efficient. For example, it is necessary to optimize the concentration of DS and FS, the flow rates of FS and DS, the pH, the temperature, and the orientation of the membrane, which can be the active layer facing FS (AL-FS) or active layer facing DS (AL-DS). Furthermore, it is important to control the characteristics and properties of the membrane such as material, mechanical and chemical stability, active area, porosity, and hydrophobicity [65].

In addition to the above, there are other relevant factors influencing the FO process that must be considered to solve possible drawbacks. Despite the wide variety of FO applications and the extensive FO-related research, there are some process issues and challenges that require still special attention for the process to maximize its commercial and industrial possibilities. These include the choice of the draw solution, the reduction in reverse salt flow, the regeneration of DS, and the reduction in concentration polarization and membrane fouling, as shown in Figure 3 [46].

To choose the possible draw solutions, it must be taken into account that they should meet a series of characteristics and requirements. Some important qualities are that it must generate high osmotic pressures [43,66], be economic, safe, and nontoxic, give minimal reverse draw solution flux, be stable, not react with the membrane material, and be easy to recover [67]. Commonly, solutes with a high solubility in water are selected to avoid their diffusion through the membrane. To improve the performance of the membrane by reducing concentration polarization on the surface of both sides, it is favorable to choose solutes with small molecular weight, giving low viscosity in the aqueous solution. Another important criterion, from an energetic point of view, is to have an easy and/or useful recovery or regeneration [67]. Extractive solutions with very varied solutes (inorganic salts, volatile compounds, organic solutes, etc.) have been suggested and studied. To date, most inorganic salt solutions as NaCl, MgCl₂, KNO₃, and MgSO₄ have been tested due to their low cost and high osmotic pressure, with sodium chloride (NaCl) frequently selected as a reference DS for several reasons. First, it is generally used for standard membrane tests allowing a comparison of the results obtained with data from the literature because NaCl is commonly used as a DS. Furthermore, seawater and reverse osmosis concentrate are widely used as DSs in several interesting applications [68]. However, there are other interesting potential inorganic DSs depending on their characteristics and applications. For example, K₄P₂O₇, KCl, and NH₄PO₃, which have the advantage of having fertilizing properties and providing high osmotic pressure, can be used as DSs if the end use of the water recovered is in irrigation. In this case, DS recovery would not be necessary [69,70], with subsequent economic savings. Organic-based solutes, compared to inorganic solutes, tend to have higher molecular weights, making their utilization somewhat more challenging. These solutes typically include sugars, diethyl ether, or organic salts. Studies have been conducted using common food additives such as monosodium glutamate (MSG), saccharin (SAS), and trisodium citrate (TSC), which generate slightly higher osmotic pressures but lower water flux than NaCl [71].

In addition, in some processes, gases such as CO₂, SO₂, and NH₃ have been used due to their good solubility in water. However, they have not been implemented in real processes due to their limited osmotic pressure and high energy consumption requirements. There are also other less developed proposals for using magnetic solutes and hydrogels, which currently make the processes more expensive and are not sufficiently understood [72,73].

At present, the choice of DS and its regeneration are key issues in the application of FO. Energy-consuming solute recovery is one of the major considerations in selecting the DS. Some regeneration methods may consist of their direct use without ulterior recovery [74]. In some cases, DS is regenerated by membrane separation, such as RO [75], NF [76], UF [77], MD [78], ED [79], chemical precipitation [80], or thermal separation [81]. Other options are magnetic recovery and electrolytic recovery. Although there are various methods for DS regeneration, each method has its advantages and limitations for the application of the FO process [51].